Titan and Enceladus – two of Saturn’s 47 known moons – are of biological interest to space scientists.
About one-10th the size of Titan, Saturn’s moon Enceladus is the brightest body in the solar system because its surface reflects almost 100 percent of the sunlight that strikes it. Enceladus has a varied terrain; some areas have craters, while other areas have none, indicating that major resurfacing events have taken place in the geologically recent past. The surface of Enceladus has fissures, plains, corrugated terrain and other crustal deformations, indicating that the interior of the moon may be liquid today. Scientists suggest that Enceladus may be heated by a tidal mechanism similar to Jupiter’s moons.
Cassini observations have revealed a water vapor plume in the southern polar region of Enceladus, hinting that this moon might have liquid water and making it a possible new candidate for astrobiological research.
As the largest moon of Saturn, Titan is the only planetary satellite in our solar system known to have a substantial and permanent atmosphere – though it is nothing like the atmosphere of Earth. The atmosphere of Titan, at a temperature of 94 Kelvin (290 degrees below zero Fahrenheit), is rich with nitrogen and methane. The interaction of this atmosphere with solar ultraviolet radiation could generate saturated and unsaturated hydrocarbons as well as nitriles, through methane photolysis. These products could condense as aerosols and reach the surface, where some could liquify while others take solid form.
Observations of Titan have indicated that it may have lakes or oceans of liquid hydrocarbons on its surface in addition to frozen deposits of these compounds. Some scientists have speculated that, over billions of years, impacts on the surface of Titan could have melted frozen surface materials, yielded water, and thus enabled chemical reactions that could possibly produce amino acids. Since prebiotic organic chemistry is considered a possibility on Titan, this planetary satellite is considered a good target for astrobiological investigation.
Observations from NASA’s Cassini Saturn Orbiter have revealed that the dark equatorial regions, previously observed on Titan and thought to be possible liquid hydrocarbon oceans, are seas of sand dunes similar to those found on Earth. The sand may have formed through bedrock erosion caused by liquid methane rain. Another theory is that the sand may come from organic solids produced by photochemical reactions in Titan’s atmosphere. Observations from the European Space Agency’s Huygens Probe during its descent through Titan’s atmosphere have shown gullies, streambeds and canyons there, along with ice pebbles and possible methane or ethane ground fog on surface. The surface is darker than originally thought, consisting of a mixture of water and hydrocarbon ice. The scientific consensus is that Titan’s methane is not a product of biological activity.